1. Field of the Invention
This invention relates to standards- or format-compliant visible watermarking techniques for digital images and video which operate on decoded transform-coefficients. The techniques can be implemented in apparatuses, methods, and programs of instructions, e.g., software.
2. Description of the Related Art
A watermark is a digital pattern embedded into the original content of a host signal, such as a digital image. A watermark can be inserted into a digital image by manipulating pixel values in the spatial domain representation or by altering certain transform coefficients in the frequency (e.g., DCT) domain representation. In the case of a block-based DCT representation, such as that used in the JPEG or MPEG compression standards, frequency coefficients in select blocks can be altered to produce a desired watermark pattern.
There are different types of watermarks which serve different purposes. One type of watermark is visibly applied to a digital image or video frame(s) to prevent or discourage unauthorized use of the image or video. The watermark may be something simple and unobtrusive, such as a small logo in a corner of the image or video frame. However, such a watermark can be easily cropped away or concealed without a drastic loss in quality. At the other extreme, the entire image may be “corrupted” in a manner that can be undone using a key. This is similar to encrypting all of the data, except that straight-forward encryption does not leave the data format-compliant.